Miniature Telephoto Lens Assembly

1. A lens assembly, comprising: a plurality of refractive lens elements arranged along an optical axis, wherein at least one surface of at least one of the plurality of lens elements is aspheric, wherein the lens assembly has an effective focal length (EFL), and wherein the lens assembly has a total track length (TTL) of 6.5 millimeters or less and a ratio TTL/EFL of less than 1.0, wherein the plurality of lens elements comprises, in order from an object side to an image side, a first lens element with positive refractive power and a second lens element with negative refractive power, wherein a focal length f1 of the first lens element is smaller than TTL/2.

2. The lens assembly of claim 1 , wherein the plurality of lens elements further comprises, in order from an object side to an image side, a third lens element with negative refractive power, and a fourth element, wherein the third and fourth lens elements are separated by an air gap which is greater than TTL/5.

3. The lens assembly of claim 2 , wherein the plurality of lens elements further comprises a fifth lens element, wherein one of the fourth and the fifth lens elements has a positive refractive power and the other of the fourth and fifth lens elements has a negative refractive power.

4. The lens assembly of claim 3 , wherein the fourth lens element and the fifth lens element are separated by a second air gap which is smaller than TTL/20.

5. The lens assembly of claim 4 , wherein a focal length f1 of the first lens element is smaller than TTL/2.

6. The lens assembly of claim 4 , wherein a lens assembly F # is smaller than 2.9.

7. The lens assembly of claim 6 , wherein the first lens element has an Abbe number greater than 50 and the second lens element has an Abbe number smaller than 30.

8. The lens assembly of claim 7 , wherein one of the fourth and fifth lens elements that has a positive refractive power is characterized by an Abbe number smaller than 30 and wherein the other of the fourth and fifth lens elements that has a negative refractive power is characterized by an Abbe number greater than 50, thereby adding compensation for chromatic aberrations and assisting in bringing all fields focal points to an image plane.

9. The lens assembly of claim 8 , wherein the third, fourth and fifth lens element are made of plastic.

10. The lens assembly of claim 8 , wherein all the lens elements are made of plastic.

11. The lens assembly of claim 8 , wherein the first lens element has a convex object-side surface and a convex or concave image-side surface and wherein the second lens element is a meniscus lens having a convex object-side surface.

12. The lens assembly of claim 11 , wherein the focal length f1, a focal length f2 of the second lens element and a focal length f3 of the third lens element fulfill the condition 1.2×|f3|>|f2|>1.5×f1.

13. The lens assembly of claim 1 , wherein the first lens element has an Abbe number greater than 50 and the second lens element has an Abbe number smaller than 30.

14. The lens assembly of claim 13 , wherein the first lens element has a convex object-side surface and a convex or concave image-side surface and wherein the second lens element is a meniscus lens having a convex object-side surface.

15. The lens assembly of claim 14 , wherein a lens assembly F # is smaller than 2.9.

16. The lens assembly of claim 15 , wherein the plurality of lens elements further comprises a third lens element with negative refractive power, a fourth lens element and a fifth lens element, wherein one of the fourth and fifth lens elements has a positive refractive power and the other of the fourth and the fifth lens elements has a negative refractive power.

17. The lens assembly of claim 16 , wherein the third, fourth and fifth lens element are made of plastic, and wherein the fourth and fifth lens elements are separated by a second air gap which is smaller than TTL/20.

18. The lens assembly of claim 16 , wherein one of the fourth and fifth lens elements that has a positive refractive power is characterized by an Abbe number smaller than 30 and wherein the other lens element of the fourth and fifth lens elements that has a negative refractive power is characterized by an Abbe number greater than 50, thereby adding compensation for chromatic aberrations and assisting in bringing all fields focal points to an image plane.

19. The lens assembly of claim 18 , wherein the focal length f1, a focal length f2 of the second lens element and a focal length f3 of the third lens element fulfill the condition 1.2×|f3|>|f2|>1.5×f1.

20. A lens assembly having a total track lens (TTL) smaller than an effective focal length (EFL) thereof and comprising: a plurality of lens elements made of at least two different polymer materials having different Abbe numbers and including, in order from an object plane to an image plane along an optical axis of the lens assembly, a first lens element having positive optical power and a pair of second and third lenses having each a negative optical power, the combination of the first, second and third lens elements providing the lens assembly with a ratio TTL/EFL <1, wherein the second and third lens elements are each made of a different one of the at least two polymer materials having a different Abbe number to reduce chromatic aberrations of the lens assembly, the plurality of lens elements further including a combination of fourth and fifth lens elements configured to assist in bringing all fields focal points to the image plane, wherein the third and fourth lens elements are separated by an air gap which is greater than TTL/5, and wherein the fourth and fifth lens elements are each made of a different one of the at least two polymer materials having a different Abbe number to further reduce chromatic aberrations of the lens assembly.